PARAM(M(J1UM 



283 



from contact with the dust of the air, do not develop Infusoria, 

 no matter how long they may be kept. This method of exclud- 

 ing Infusoria and other minute organisms, especially bacteria, 

 is emploj^ed to-daj^ in canning meat, vegetables, and fruit. 

 The experiments referred to gave a death-blow to the theory of 

 spontaneous generation, and led to the conclusion that all 

 Protozoa are derived from living 

 germs. 



Whence the living germs come 

 which enter the water it is not 

 difficult to determine. Many 

 Protozoa can pass into a quiescent 

 " spore " stage in which they may 

 be dried and blown about without 

 loss of life. Dry grass, strav/, 

 and other substances contain 

 some of the germs, and others 

 float in the air and fall as dust 

 into the water. Even drinking 

 water may contain here and there 

 an infusorian or its germ. When, 

 therefore, one fills a clean vessel 

 with pure water, and puts hay or 



dry leaves in it, and lets it stand open to the air in a warm 

 place, it is pretty certain that there will be germs in the mixture. 

 The heat and the organic infusion merely facilitate their devel- 

 opment. 



The reproductive capacity of Protozoa is so great that their 

 importance in the world, despite their small size, is not 

 astonishing. One of the early students of Protozoa, Ehren- 

 berg, computed that from one individual of Paramoecium 



Fig. 269. — Louis Pasteur, dis- 

 coverer of the importance of 

 bacteria in tlie air for disease 

 and fermentation. 



